1. Field of the Invention
The present invention generally relates to image forming apparatuses and, more particularly, to an image forming apparatus that is capable of forming an image by a recording liquid while conveying a paper in a transverse direction.
2. Description of the Related Art
Inkjet recording apparatuses are known as image forming apparatuses incorporated in a printer, a facsimile machine, a copy machine and a combination machine of a printer, a facsimile machine and a copy machine that perform image formation (recording, printing and copying may be used as equivalent words) by causing droplets of a recording liquid (hereinafter, may be referred to as ink droplets) to adhere onto a paper, while conveying the paper, using a recording head (image forming means) constituted by a liquid droplet discharge head which discharges droplets of a recording liquid.
As the liquid droplet discharge head, there are used a piezoelectric type using an electromechanical transducer such as a piezoelectric element, a thermal type which generated bubbles by film boiling of ink using an electrothermal transducer such as a heating resistor provided in a discharge chamber, and an electrostatic type which discharges ink droplets by deforming by an electrostatic force a diaphragm forming a wall of a discharge chamber.
In the meantime, since ink is caused to adhere onto a paper according to an inkjet recording method, there occurs a phenomenon that the paper extends due to a water component contained in ink when image is formed on the paper. This phenomenon is referred to as cockling. When the cockling occurs, the paper is waved, which results in changes in a position of the paper surface and a nozzle plane of the head location to location. If a degree of cockling increases, in a worst case, the paper is brought into contact with the nozzle plane of the head, which causes contamination of the nozzle plane of the head and contamination of the paper itself. Additionally, landing positions of the ink droplets may be offset due to influence of the cockling.
By the way, in a conventional inkjet recording apparatus, for example, as shown in FIG. 1, a convey roller 601 and a press roller 602 are located on an upstream side and a paper eject roller 604 and a spur 603 are located on a downstream side with a printing area by an inkjet head 600 located therebetween, and a second paper eject roller 606 and a second spur 605 are located on a downstream side of the paper eject roller 604 and the spur 603, and, further, a guide member 507 is located in the printing area so as to guide the paper so as to perform conveyance by two sets of the roller pair.
In this case, since there is a demand for increasing the image forming area, in order to acquire a large printing area, there exist an inkjet recording apparatus that performs printing in a state where an accuracy of feed of a paper cannot be guaranteed, that is, in a state where a paper is engaged with only one pair of rollers. However, in the state where a paper is engaged with only one pair of rollers, if a paper lift occurs, an accuracy of feed cannot guaranteed since there is no measures to solve the paper lift or a force to convey the paper cannot be acquired, and, thus, there is a problem in that an image quality deteriorates.
Additionally, as disclosed in Japanese Patent Publication No. 2897960 and Japanese Laid-Open Patent Applications No. 7-53081 and No. 2003-103857, there is suggested an inkjet recording apparatus having an endless charge belt that is capable of maintaining excellent flatness by preventing a paper from being lifted from the charge belt by conveying the paper by rotating the charge belt in a state where the paper is electrostatically attracted to the charge belt of which surface is electrically charged.
Additionally, although it is an image forming apparatus using an electrophotography, as disclosed in Japanese Laid Open Patent Application No. 10-10878, there are known a corona discharge type, an endless belt type and a separation claw type as a separation system for separating a transfer material electrostatically attached to a conveyor.
Further, as disclosed in Japanese Laid-Open Patent Application No. 10-138587, it is known that a paper after image formation is pressed by a spur when it is ejected in an inkjet recording apparatus.
By the way, generally, there is a carding mesh in a paper, and the carding direction coincides with a longitudinal direction of the paper. If a such a paper having a carding mesh is conveyed in a direction in which the carding direction is perpendicular to a conveying direction, the nozzle plane and the paper may contact with each other since the paper bends largely in a direction along the conveying direction due to cockling caused by adhesion of liquid droplets as mentioned above, which results in a problem peculiar to an inkjet recording apparatus that an image failure occurs.
That is, in a case where a longitudinal mesh direction conveyance is performed in which the carding direction of the paper 500B coincides with the conveying direction as shown in FIG. 2A, if cockling occurs in the paper, the paper merely waves in a direction (main-scanning direction) perpendicular to the conveying direction as shown in FIG. 2B. Thus, an amount of waving as a whole is relatively small, and, thereby, the paper 500B is prevented from being brought into contact with a nozzle plane of a head 600.
On the other hand, in a case where a transverse mesh direction conveyance is performed in which the carding direction of the paper 500A is perpendicular to the conveying direction as shown in FIG. 3A, the paper 500A is lifted in a curled shape due to cockling as shown in FIG. 15B or FIG. 1, and, thus, there is an extremely high possibility that the paper is brought into contact with the nozzle plane of the head 600.
Therefore, in the conventional inkjet recording apparatus, only the setting of papers causing the longitudinal mesh direction conveyance is permitted, and the setting causing the transverse mesh direction conveyance is prohibited so that the transverse mesh direction conveyance is not performed. For example, papers of A4 size are used, only the setting causing the longitudinal direction of A4 size to be a conveying direction is permitted, and the setting causing the transverse direction of A4 size to be the conveying direction is prohibited. The above-mentioned problem does not occur in an electrophotography image forming apparatus using no recording liquid.
However, it has been demanded for an inkjet recording apparatus to be capable of handling large size papers, and is required for the inkjet recording apparatus to use not only A4 size papers as in conventional apparatus but also A3 size papers. In such a case, since the shorter side width of A3 size is equal to the longer side width of A4 size, it is convenient for a user if the A4 size papers can be set in either the longitudinal mesh direction conveyance or the transverse mesh direction conveyance.
Moreover, in the case of an inkjet recording apparatus, a printing speed can be increased as a number of times of feed in a sub-scanning direction is decreased, that is, an area, which is printed by a single sub-scanning, is increased. Thus, if A4 size papers are used, the printing speed for the transverse mesh direction conveyance can be higher than that for the longitudinal mesh direction conveyance.
However, in the conventional inkjet recording apparatus, there is a problem that papers are not permitted to be conveyed in the transverse mesh direction due to the problem of cockling peculiar to the inkjet recording as mentioned above.